This study evaluated the combined effects of biochar and straw on N₂O flux and the community compositions of nitrifiers and denitrifiers in the maize season in an intensively farmed area in northern China. The experiment consisted of four treatments: (1) CK (only chemical fertilizer application); (2) C (biochar application); (3) SR (straw application to the field); and (4) C+SR (the application of both biochar and straw). The results indicated that during the maize growing season, N₂O flux decreased by 30.3% in the C treatment and increased by 13.2% and 37.0% in the SR and C+SR treatments compared with CK, respectively. NO₃⁻-N, NH₄⁺-N, and microbial biomass carbon (MBC) were the main soil factors affecting N₂O flux, and they were positively correlated with NO₃⁻-N and negatively correlated with MBC in the C treatment and positively correlated with NH₄⁺-N in the SR and C+SR treatments. Both biochar addition and straw return shifted the community compositions of nitrifiers and denitrifiers. N₂O production was mainly reduced by promoting the ammonia-oxidizing bacteria (AOB) gene abundance and inhibiting the nirK gene abundance in the C treatment but promoted by inhibiting the AOB and nosZ gene abundances in the SR and C+SR treatments. Nitrosospira (AOB) and Rhizobium (nirK) were the main contributors among the treatments. NO₃⁻-N, NH₄⁺-N, and MBC were the main soil factors affecting the denitrifier communities. The predominant species associated with the nirK, nirS, and nosZ genes were positively correlated with NO₃⁻-N and MBC and negatively correlated with NH₄⁺-N. These results provide valuable information on the mechanism of N₂O production and reduction in biochar- and straw-amended soil under field conditions.